The pathology and biology of selected experimentally-induced and naturally-occurring neoplasms and neoplastic-related diseases of rodents were studies in order to elucidate their pathogenesis, including mechanisms of disease. The origin and pathology of mouse lung tumors, in particular so-called Clara cell papillary tumors, were studied with serial sections, immunocytochemistry, histochemistry and electron microscopy. After a detailed analysis in two strains of mice, conclusive evidence was presented that virtually all N-nitrosoethylurea (ENU)-induced lung tumors in mice were of alveolar Type II cell origin; none were of Clara cell origin. These findings have great implications for mouse lung tumor classification since many recent authors have inadvertently joined the bandwagon of Clara cell tumor terminology without conclusive evidence of the origin of these papillary tumors. Retroviral antigens were localized in human, simian and murine fixed tissue sections from cases of AIDS of leukemia using polyclonal and monoclonal antibodies. This technique has allowed a major advance in understanding the neurologic complications of acquired immune deficiency syndrome (AIDS) by identifying specific central nervous system (CNS) cell types infected with human immunodeficiency virus (HIV).